Communication networks are used in a very wide range of areas to control work sequences in a decentralized manner. In industrial automation systems in particular it is particularly important for the automatic work processes to be coordinated with one another. This is achieved in that the individual communicating network nodes in the communication network have internal clocks, with synchronization messages being transmitting to synchronize all the internal clocks. The internal clock of a respective network node hereby operates at a corresponding node clock frequency, which may in some instances be different for the individual network nodes. The clocks are synchronized on the basis of a predefined synchronization clock frequency, with the synchronization messages being transmitting as a function of the synchronization frequency. This means that synchronization messages are sent at fixed clock intervals according to the synchronization clock frequency. The individual synchronization messages transmitted in the communication network contain the pulse counter status of the synchronization clock. Each network node updates this pulse counter status by estimating the number of pulses of the synchronization clock between transmission of the synchronization message at the preceding network node and receipt of the synchronization message at the respective network node. This estimate is generally carried out by estimating the pulse ratio (also referred to as the Rate Compensation Factor RCF) between the synchronization clock frequency and the node clock frequency of the respective network node. The estimated pulse ratio can then be used to convert the time interval between receipt of a synchronization message at the preceding network node and receipt of the synchronization message at the respective network node, measured in node clock frequency pulses, to synchronization clock frequency pulses. The resulting number of pulses is then added to the pulses of the received synchronization message and a correspondingly updated synchronization message is sent out again from the corresponding network node. It is problematic here that fluctuations can occur in the synchronization clock frequency, which result in an imprecise estimation of the pulse ratio and therefore incorrect determination of the pulse counter status of the synchronization clock in the individual network nodes.
The Profinet standard is known from the prior art in the field of industrial automation, this being an Ethernet which meets industrial requirements. This standard operates according to the principle set out above, according to which the pulse counter statuses in the synchronization messages are updated in the network nodes. Profinet-based systems generally use the standard IEEE 1588 to synchronize the internal clocks of the network nodes, said standard updating the pulse counter statuses of the synchronization messages according to the principle set out above. According to this standard synchronization messages are transmitted in succession from one network node to the next in a logical sequence or tree structure. The synchronization messages originate from a master element, which is the first element in the sequence or in the tree structure. The synchronization messages originally contain a time stamp of the counter of a synchronization clock in the master element when a synchronization message was transmitted. The network nodes in the sequence or tree structure process and forward this information. A network node here adds all the estimated time delays between transmission of a synchronization message from the preceding network node and its own transmission of the synchronization message as content to the synchronization message.
As set out above it is problematic with the communication networks known from the prior art that when the synchronization clock frequency of the synchronization clock is changed, errors occur in the determination of the pulse counter status in the network nodes, which in turn results in imprecise synchronization of the internal clocks of the network nodes.